Microporous filter media are employed for removing ultra-fine microscopic particles from the medium in which they are suspended. As examples of particulate media which may be separated by such filters from the fluid environment in which they are disposed, mention may be made of bacteria, colloidal substances and the like.
Surface-type filters capable of effectively filtering materials of a relatively small size suffer from a deficiency of lack of uniformity of pore size, non-uniformity of spatial distribution of pores, fragility as the pores become more uniformly distributed and a relatively low percentage of open area. The term "open area" as used herein is intended to refer to the voids or pores or openings in the filter unit as a percentage of the total surface area of the filter. It will be readily seen that the efficiency of a filter would increase with an increase in open area.
The term "surface-type filters", as used herein is intended to refer to thin films having pores which extend from one surface to the other surface and function by trapping material on the upstream side of the filter. This is in contradistinction to depth-type filters which are relatively thick, possess pores of significant length, and function by trapping materials within the filter element.
In selecting a material for construction of a filter, consideration must be given to the environment in which it is to function. Thus, the material must not be significantly effected by the corrosive, solvating action or other conditions of the environment which would subject the filter to severe damage or destruction. Metal filters are particularly useful because of their strength and the varieties of metals available to permit a wide selection of filter material depending upon the environment in which they are to be employed. Generally, metal filters are formed from sintered particles in the order of 2-50 microns in diameter. Such a sintering operation requires controlled melting to fuse the particulate mass sufficiently to bind the particles together but not to the extent that a passage through them is totally restricted. It will be seen that unless this method is carefully controlled, the resulting filter may have a relatively low through-put or possess a non-uniform pore size.
A novel metal filter and methods for obtaining such a filter has now been found which are not susceptible to the deficiencies of the prior art.